The present invention relates generally to an apparatus for degassing, submerging, agitating and pumping molten metal. More particularly, the present invention relates to a mechanical apparatus for moving or pumping molten metal such as aluminum, zinc or magnesium. Specifically, the present invention is related to a drive for such an apparatus in which a motor is positioned above a molten metal bath and rotates a vertical shaft. The lower end of the shaft drives an impeller or a rotor to impart motion to the molten metal.
In the processing of molten metals, it is often necessary to pump molten metal from one place to another. When it is desired to remove metal from a vessel, a so-called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so-called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so-called gas injection pump is used. In each of these pumps, a rotatable impeller is submerged, typically within a pumping chamber, in the molten metal bath contained in the vessel. Additionally, the motor is suspended on a superstructure over the bath by posts connected to the base. In another embodiment of these pumps, a rotatable impeller can be submerged in the molten metal bath by a shaft affixed to a suspended motor, where the motor is not supported over the bath by any posts. Rotation of the impeller within the pumping chamber forces the molten metal as desired in a direction permitted by the pumping chamber design.
Mechanical pumps for moving molten metal in a bath historically have a relatively short life because of the destructive effects of the molten metal environment on the material used to construct the pump. Moreover, most materials capable of long term operation in a molten metal bath have relatively poor strength which can result in mechanical failure. In this regard, the industry has typically relied on graphite, a material with adequate strength, temperature resistance and chemical resistance, to function for an acceptable period of time in the harsh molten metal environment.
While graphite is currently the most commonly used material, it presents certain difficulties to pump manufacturers. Particularly, mechanical pumps usually require use of a graphite pump housing submerged in the molten metal. However, the housing is somewhat buoyant in the metal bath because the graphite has a lower density than the metal. In order to prevent the pump housing from rising in the metal and to prevent unwanted lateral movement of the base, a series of vertical legs are positioned between the pump housing and an overhead structure which acts simultaneously to support the drive motor and locate the base. In addition to functioning as the intermediate member in the above roles, the legs, or posts as they are also called, must be strong enough to withstand the tensile stress created during installation and removal of the pump in the molten metal bath.
Similarly, the shaft connecting the impeller and the motor is constructed of graphite. Often, this shaft component experiences significant stress when occluding matter in the metal bath is encountered and sometimes trapped against the housing. Since graphite does not possess as high of a strength as would be desired, it would be helpful to reinforce the leg and shaft components of the pump. A shaft or post assembly made entirely of ceramic would be brittle and subject to an unexpected failure. Furthermore, exposed metal components residing in the molten metal bath can dissolve.
In addition, graphite can be difficult to work with because graphite has different thermal expansion rates in its two grain orientations. This may result in a post and base having divergent and conflicting thermal expansion rates in the molten metal environment. This problem is compounded by the fact that pump construction has historically required cementing the graphite post into a hole in the graphite base. This design provides no tolerance between the components to accommodate this divergent thermal expansion. Unfortunately, this can lead to cracking of the base or the post. Accordingly, it would be desirable to have a molten metal pump wherein the mating of a post and a base is achieved in a manner which accommodates divergent thermal expansion tendencies.
Tensor pumps combine ceramic and steel with an improved design to increase reliability of molten metal pumps. Traditional graphite pumps must be rebuilt about three times per year when the graphite posts become oxidized or broken. By replacing the posts with a stronger material that will not oxidize, the life of the pump has been extended to up to a year. Fewer rebuilds mean lower costs and less labor to maintain pumps.
Tensor pumps are centered around a high temperature alloy steel rod loaded in tension. The tensor rods hold the ceramic posts in compression to maximize their strength. Fragile ceramic sleeves are no longer needed to protect graphite posts from oxidation and abrasion. Ceramic sleeves can also hide post oxidation which could result in unexpected failures. Not only are the ceramic posts stronger than graphite, if they do break, the base is still supported by the steel rods.
In addition, tensor pump bases are very strong. The holes in the base are much smaller which means that less material needs to be removed during manufacturing. The base, like the posts, is also loaded in compression for maximum strength, which can last up to a year between rebuilds and which replaces graphite posts with ceramic and steel.
Tensor pumps are also the most economical molten metal pumps available. With a reasonable initial cost, predictable parts usage, and low energy requirements, tensor pumps cost less to operate than other mechanical pumps. And when a pump does need maintenance, it can be pulled out of service and replaced with another pump quickly, minimizing the impact on production. When the pump finally does need to be rebuilt there are no structural cement joints, which is another advantage of tensor pumps. The pump is bolted together and it can actually be used right after assembly. Tensor pumps can be made as transfer, circulation, and gas injection pumps. Flows range from a minimum of 100 pounds (45 kg.) aluminum per minute to a maximum of 30,000 pounds (13.6 tons) per minute.
The prior art utilized a metal rod having a foot welded to the base and which would be trapped in the graphite base of the molten metal pump. The present invention. Instead of having to weld the foot onto the rod, there is now a keeper pair which is easily put into place at the groove of the rod. Then when the rod is pulled up, the two halves of the keeper pair catch and engage the retainer cup and work in the same way as the prior design.